The invention relates generally to the manufacture of devices such as semiconductors and more particularly to a print quality test structure for lithographic device manufacturing.
The process of manufacture of devices such as semiconductors includes processing of a silicon wafer by the repetition of a series of procedures including: oxidation, deposition, lithography, etching, implanting and diffusing. Tools used in the lithography stage generally include step and repeat type alignment and exposure apparatuses, called steppers, which have been used in device manufacturing. As is well known in the art, the exposure apparatus of this type is arranged such that an image of a pattern formed on a reticle is projected by a projection lens, in a reduced scale, onto a semiconductor wafer placed under the projection lens, while the semiconductor wafer is moved stepwise or intermittently, whereby discrete areas on the wafer are sequentially exposed to the images of the pattern of the reticle with a radiation such as light. By this, images of the pattern of the reticle are respectively transferred onto the discrete areas of the wafer.
Alternatively, other methods of transferring the images could be used. For example, a 1x lens could be used (no reduction). As well, a holographic apparatus could be used.
Depending on the level of production, the wafer may be inspected for lithography print quality (to detect defects) between the application of each layer. Intensive defect control during device manufacturing is important in order to maintain high print quality.
There are a number of methods and apparatuses available to assess print quality of the actual device. Scanning electron microscopy (SEM) analysis is one common method used. SEM is a method for high-resolution imaging of surfaces. The SEM uses electrons for imaging, much as a light microscope uses visible light. An incident electron beam is raster-scanned across the sample""s surface, and the resulting electrons emitted from the sample are collected to form an image of the surface.
SEM analysis is costly and time consuming. It also generally requires surgery on a completed device, such as removing the metal, conducting microsurgery, cutting the device apart or polishing it.
Manufacturers of lithographic devices commonly create test patterns in order to assess the print quality. The test patterns are kept away from the working device since when the wafers are cut into chips, these test structures that were created lithographically are to be thrown away. There is no record of the print quality.
Other current methods of print quality assessment known in the art suffer from a variety of problems. The problem of tracing print quality to a critical dimension measurement in fabrication is difficult, if not impossible, on an individual device. Critical dimension tests must be done where the critical dimensions of the line are measured. Devices also undergo a separate focus test as well. And since there is no way to know ahead of time whether a device is good or bad, every device must be tested. Since testing procedures are generally about 4 or 5 hours per device, this results in a great deal of wasted time and cost.
These methods also suffer from the problem that they can not detect defects which are invisible from the surface. Devices are manufactured layer by layer and the layers themselves may get covered during fabrication of the device. For example, a layer of metal that overlaps everything printed below it may be used, no longer allowing for visual inspection.
Screening for defects is also a problem because in lithographic processes, it is impossible to ensure that every die on a single wafer is perfect everywhere. It is also impossible to inspect every die to ensure that it is perfect or even to assess the wafer in a proper manner. It would be too time consuming.
Current methods also do not decouple lithographic effects such as over exposure, under exposure, focus, 90 and 45 degree astigmatism.
Finally, other methods do not provide a permanent record of the print quality on the device.
Therefore, there exists a need for a print quality test structure for lithographic device manufacturing that overcomes the problems of current print quality assessment.
The invention is directed to a print quality test structure for lithographic device manufacturing.
In one embodiment of the invention, there is provided a method of forming a lithographic device by a lithographic process. The process involves the projection and focussing of radiant energy passing through a reticle having a selected pattern thereon such as to expose photoresist material on a wafer surface to the selected pattern. The reticle also has a print quality configuration thereon capable of producing a test structure on the wafer surface in close proximity to the selected pattern or a selected portion thereof. The test structure is capable of being visually inspected to indicate the quality of the selected pattern which is formed on the wafer surface. The print quality configuration comprises a series of characters incrementing in size according to a binary pattern to a resolution exceeding the resolution capabilities of the lithographic process.
In another embodiment, there is provided a method of inspecting the print quality as defined by the exposure and the focus of the print of a lithographic device such as a semiconductor. The method comprises the step of visually inspecting the selected test structure on the wafer surface to determine the number of printed and visible dots relative to the number of printed and visible holes whereby to give an indication of print quality.
In another embodiment, there is provided a device manufactured by a lithographic process involving the projection and focussing of radiant energy passing through a reticle such as to expose photoresist material on a wafer surface to a selected conductor pattern, the reticle having a print quality configuration capable of producing a selected test structure on the wafer surface. The device comprises a wafer defining a surface; at least one layer on the wafer surface including at least one critical element in said selected conductor pattern, the critical element having a print quality dependent on the exposure and focus of the print; and a selected test structure providing a permanent visual indication of the print quality of the critical element fixed to a portion of the wafer surface adjacent the at least one critical element.
In another embodiment, there is provided a reticle used in the formation of a lithographic device such as a semiconductor by a lithographic process involving the projection and focussing of radiant energy passing through the reticle such as to expose photoresist material on a wafer surface to a selected conductor pattern. The reticle is characterized in that the reticle is provided with a print quality configuration capable of producing a selected test structure on the wafer, the test structure being capable of being visually inspected to indicate the quality of the formation of the device on the wafer surface.
In another embodiment, there is provided a step and repeat exposure apparatus used in the formation of a lithographic device comprising: a light source for providing radiant energy; a stage for supporting a wafer in a position to receive the radiant energy; and a reticle located intermediate the light source and the stage such that in use of the apparatus the radiant energy passes through the reticle such as to expose photoresist material on the wafer to a selected conductor pattern, the reticle being provided with a print quality configuration capable of producing a selected test structure on the wafer surface, the test structure being capable of being visually inspected to indicate the print quality of the selected pattern which is formed on the wafer surface.
Many advantages of the present invention have been identified, including:
Requiring only good devices to undergo tests, such as a focus test, thus allowing screening of devices to be more time and cost efficient;
Allowing the ability to trace print quality to a critical dimension measurement in fabrication;
Allowing the print quality to be assessed at any stage of manufacture;
Ability to decouple lithographic effects such as over exposure, under exposure, focus, 90 and 45 degree astigmatism;
Ability to leave a permanent visible record of the print quality;
Allowing use of the invention in other technologies to which a lithographic technique can be used such as HBTs, MESFETs, SAWs; and
Eliminates the human errors resulting from current visual inspection methods.